The present invention relates to a two-group zoom lens system of telephoto type (hereunder referred to as a "zoom lens system of two-group type") that is characterized by a simple mechanical structure. More particularly, the present invention relates to a zoom lens system that has a smaller backfocus than lens systems for a conventional single-lens reflex camera and that is suitable for use with a compact camera.
Examples of conventional two-group type zoom lens systems are described in Japanese Patent Laid-Open Publications No. SHO 56-128911, No. SHO 57-201213, No. SHO 60-48009, No. SHO 60-191216, etc.
However, all of those conventional zoom lens systems have a small backfocus, so the lenses in the second lens group are too bulky to achieve compactness when the lens system is incorporated into a camera. A further problem with those lens systems is that internal reflections tend to occur between the film surface and the last lens surface.
Japanese Patent Laid-Open Publications No. SHO 62-90611, No. SHO 64-57222, No. SHO 62-113120 and No. SHO 62-264019, disclose two-group type zoom lens systems that have a comparatively large backfocus in spite of their intended use with compact cameras However, all of these zoom lens systems have a half view angle of about 30 degrees at the wide-angle end hence are not suitable for the purpose of taking scenic and other pictures at wide angles.
Japanese Patent Laid-Open Publication No. SHO 63-276013 proposes a zoom lens system that uses a negative lens as the first lens element. However, this negative lens has a distribution of refractive index, so the zoom lens system under consideration cannot be manufactured at low cost in spite of the small number of the lens elements used. A further disadvantage is that the half view angle is as small as 30 degrees.
The most important consideration for wide-angle lenses is how to insure the marginal illumination when the stop diaphragm is effected. Conventional two-group type zoom lens systems are focused by two methods; in one method, a stop diaphragm is provided within the first lens group and the latter is moved for focusing together with the diaphragm; in the second method, a stop diaphragm is provided between the first and second lens groups and only the first lens group is moved for focusing while the diaphragm and the second lens group are fixed. The second approach realizes a system that is mechanically the simpler; However, the height of marginal rays that pass through the diaphragm position at a maximum view angle decreases to make it difficult to insure adequate marginal illumination when the stop diaphragm is effected. If focusing is effected by moving the first lens group altogether regardless of the diaphragm position, significant undercompensation for astigmatism and curvature of the field will occur at near distance.
Japanese Patent Laid-Open Publication No. HEI 1-189620 concerns the focusing of two-group type zoom lens system. However, the zoom lens system described in this patent also has a small half view angle of only about 30 degrees at the wide-angle end and is incapable of assuring adequately wide angles.